The major histocompatibility complex (MHC) gene products play a critical role in regulating the cellular immune response. In particular, the appropriate constitutive and inducible expression of class II MHC molecules is essential for normal immune response, whereas aberrantly high or low expression has been correlated with various autoimmune diseases (P. T. Massa et al., Proc. Natl. Acad. Sci. USA 84, 4219-4223 (1987)) and a type of severe combined immunodeficiency diseases (SCID) known as the Bare Lymphocyte Syndrome (BLS) (C. Griscelli et al., Immunodeficiency Rev. 1, 135-53 (1989)). Additionally, the class II major histocompatibility (MHC) antigens are known to play a crucial role in causing organ transplant rejections.
The primary regulation of both constitutive and IFN-.gamma.-induced class II MHC genes occurs at the transcriptional level. F. Figueiredo et al., J. Immunol. 143, 3781-3786 (1989). Expression of the recently identified MHC class II transactivator, CIITA, closely parallels that of class II MHC gene expression. V. Steimle et al., Cell 75, 135-46 (1993). It has also been shown that CIITA is induced by gamma interferon, and that transfection of CIITA alone into cells is sufficient to activate class II MHC, Ii, and DM genes. See, e.g., C. K. Chin et al., Immunity 1, 679 (1994); C. H. Chang et al., J. Exper. Med. 180, 1367-1374 (1994); V. Steimle et al., Science 265, 106-08 (1994). CIITA transcript is expressed constitutively in class II MHC-positive cells; however, it can be induced in certain cell types such as fibroblasts, macrophages, and glioblastoma cells upon treatment with interferon-.gamma.. See Chang et al., supra; Steimle et al., supra. The kinetic of CIITA induction by interferon-.gamma. precedes the induction of class II MHC transcripts and introduction of CIITA alone into a number of cell types is sufficient to activate class II MHC genes. However, the mode of action of CIITA is not well understood.
The N-terminal of CIITA contains an acidic domain (amino acids 30-160), followed by domains rich in proline (amino acids 163-195), serine (amino acids 209-257), and threonine (amino acids 260-322). Steimle et al., 1993, supra. An acidic domain has been found in many transcription factors and has been shown to interact with basal transcriptional machinery in vitro and in vivo. See, e.g., Schmitz et al., J. Biol. Chem. 270, 7219-7226 (1995); Tong et al., Proc. Natl. Acad. Sci. USA 92, 3259-3263 (1995). However, it is likely that the acidic domain alone is not sufficient to activate the class II MHC promoter in CIITA, and that the acidic domains of other transcription factors behave differently from the CIITA acidic domain. H. Zhou et al., Immunity 2, 545-553 (1995). Analysis of the primary amino acid sequence of CIITA does not show any homology to known conserved DNA-binding motif of transcription factors, and in vitro translated CIITA apparently does not interact with DNA (Steimle et al., 1993, supra).
Proline-, serine-, and threonine-rich domains have been found in many transcriptional factors and may have a role in protein-protein interaction. H. Zhou et al., Mol. Cell. Biol. 14, 7013-7024 (1994); T. K. Kim and R. G. Roeder, Proc. Natl. Acad. Sci. USA 91, 4170-4174 (1994). However, the role of these and other distinctive domains in CIITA has heretofore been unknown.